Most computer-driven printing devices which generate hard copy, such as laser, dot-matrix and ink-jet printers, print in a binary fashion--the output medium is divided into an array of picture elements or pixels and the devices can either print a small colored dot at each pixel location or leave the pixel location blank. In the case of monochrome printers, all of the dots are printed with a single color, whereas with color printers a dot color is chosen from a small set of colors. In any case, each dot has a generally uniform color so that the resulting output consists of an array of colored and blank pixels.
Text images are typically printed as hard copy characters in either black or white text. Pictorial images, such as those produced by photographic techniques or by computerized imaging systems, by contrast, are continuous in tonality. If a monochrome image is divided into pixels, each pixel exhibits a gray scale color whose tonal value falls within a range of tonal values. Similarly, if a color image is divided into pixels, each pixel exhibits a hue and an intensity which fall in ranges. In order to reproduce such continuous-tone images by means of electronic printing, the images must be converted into a form which is suited to the characteristics of the printing device, e.g., generally a binary format. This conversion process, which may take many forms, is generically referred to as halftoning. Although a halftone image actually consists solely of a spatial pattern of binary pixels (colored or blank dots), the human visual system integrates this pattern to create an illusion of a continuous-tone image.
During the printing process, the image to be printed is divided into a series of pixels and the value of the image in each pixel is quantized to produce a multi-bit digital word which represents the tonal value of the pixel. The image is thus converted to a stream of digital words which are provided to the printing device. In order to convert the format of each word into a format suitable for reproduction on the digital device, halftoning is performed on the digital word stream during a process called preprocessing. Numerous halftoning techniques have been developed and refined over the years. In their simplest form, such techniques compare the value of each digital word with a threshold level, and generate a binary output pixel value depending on the relative values; this simple type of "threshold" processing is also typically performed on text images.
For example, a digital scanner processing a monochrome image generates a stream of multi-bit words representing the detected light intensities. The numerical value of these words typically ranges from 0 to 255 corresponding to a 256-level gray scale or an 8-bit word. If such a digital word stream is to be reproduced on a binary printing device, the simple thresholding process typically compares the scanner output words with a single (threshold) value to produce the required binary output pixel stream. Illustratively, the fixed threshold value may be 128 for a gray scale value range between 0 and 255. In such a system, each 8-bit scanner word has effectively been compressed into a single-bit output word.
Color images are typically processed by separating each color into one or more color components or "primaries" whose superposition generates the desired color. Generally, three primary colors (either the conventional additive primary colors, i.e., red, green and blue, or the conventional subtractive primary colors, i.e., cyan, magenta and yellow) are used. A digital scanner processing a continuous-tone color image generates a stream of multi-bit words for each of the three color components. The numerical value of these words also ranges from 0 to 255, corresponding to 256 intensity levels or an 8-bit word. Thus, each colored pixel is represented by three 8-bit words or 24-bits total.
The digital word stream corresponding to a colored image is halftoned by comparing the 8-bit word for each color component with a threshold value in the same manner as monochrome processing. The color components are processed separately so that the three 8-bit scanner words are compressed into a 3-bit output word which is eventually printed as three dots--each dot being printed in one of the primary colors.
When processing a monochrome or colored mixed-mode document, it is preferable to distinguish between those areas that are image and text, and to process them differently. For example, threshold processing of text improves the clarity of the printed characters, while halftoning operations increase the reproduction quality of images printed on a document. However, distinguishing between text and image areas is rather difficult, particularly for documents with overlaying portions of these areas. The present invention is applicable to such documents and is concerned with distinguishing between text and image areas.
Therefore, it is among the objects of the present invention to provide an apparatus and a method of improving the quality of images and text produced by a binary printing device.
Another object of the invention is to provide a method and apparatus which distinguishes between areas of text and images when processing a monochrome or colored mixed-mode document for printing on a binary printing device, such as an ink jet printer or a laser printer.
Still another object of the invention is to provide such a method which can be implemented relatively easily either in specialized hardware or in existing printer drivers. Other objects will, in part, be obvious and will, in part, appear hereinafter.